The present invention relates to a toner density control device in an electrophotographic copying apparatus, and more particularly, to a device to control the density of toner held in a developing device which develops an electrophotographic latent image formed on the surface of a photoreceptor into an actual toner image.
In a conventional electrophotographic copying apparatus, a composition is generally employed wherein a developing device is disposed closely to a photoreceptor drum which rotaes in one direction, and an electrostatic latent image is developed by causing the toner contained in developer consisting of the toner and carrier placed in the developing device to be electrostaticaLly attracted with respect to an electrostatic latent image formed on the surface of a photoreceptor drum. An example of the above toner is the powder of mean grain diameter from 1 to 30 .mu.m, preferably from 5 to 25 .mu.m, which contains coloring agent and binding resin as its main constituents and additives such as charge control agent and offset inhibitor. Examples of the above carrier include glass beads; non-coated iron powder such as iron oxide powder and unoxidated iron powder; and coated magnetic powder of magnetic materials such as iron, cobalt, nickel, ferrite and magnetite which are coated with polymer such as acryl-based polymer, fluorine-based polymer and polyester, and they are normally from 50 to 2000 .mu.m in diameter.
In a developing device having a composition such as above, it is required to maintain the density of the toner contained in the developer at an appropriate level to obtain proper development of electrostatic images. To satisfy this requirement, there has been provided a toner density control device (refer to the Japanese unexamined utility model publication No. 1982-74447) in which the developer having a predetermined toner density is supplied into a developing device and agitated, the toner density of the developer contained in the developing device is detected under such condition, and the value detected as above is maintained as the control standard value, so that the replenishment et cetra of the toner is caused to be accomplished when the toner density becomes lower than the control standard value of above.
In the toner density control device of the above composition, it may seem possible to accomplish precise toner density control by supplying a developing device with a developer having a predetermined toner density, but is is not so in practice because the amount of electrostatic charge of the carrier will not be stabilized until a considerably long time has passed after the developer is supplied into the developing device. If the density of the toner is controlled by using the control standard value, under the condition that the amount of electrostatic charge of the carrier is not stabilized, the density of an image will be caused to change following the change of the amount of electrostatic charge of the carrier.
To describe it more in detail, as shown in FIG. 3 in which the quantity of the developer is 1 kg, and the initial toner density when the developer is supplied into the developing device is 6 wt %, where mean diameter of the toner particle is about 11 .mu.m and resistivity of the toner is about 1.8.times.10.sup.11 ohm.multidot.cm, and mean diameter of the ferrite carrier particle is aobut 80 .mu.m and resistivity of the carrier is about 8.0.times.10.sup.10 ohm.multidot.cm. There is a large amount of the electrostatic charge of the carrier during the initial stage after the developer is supplied into the developing device. If the toner is caused to be electrostatically attracted on an electrostatic latent image formed on the surface of the photoreceptor drum under such condition, the image density tends to reduce when the electrostatic latent image is developed into an actual image because it is difficult for the toner to be separated from the carrier. If in this case an attempt is made to obtain a proper image density, it is necessary to establish a toner density as the control standard value which is higher than the toner density of the developer initially supplied into the developing device. By such arrangement, it becomes possible to replenish the shortage of the toner in the developer and maintain a high toner density so that an image of proper density can be obtained under a condition where a large amount of electrostatic charge of the carrier exists. If, however, a high toner density is maintained as the control standard value, inconvenience results when the amount of electrostatic charge of the carrier is reduced with the lapse of time and has come to be stabilized. This is because the toner density becomes too high conversely and so will the density of the obtained image becomes high, causing possible fog.
It can be considered possible to use the toner density as the control standard value after the amount of electrostatic charge of the carrier has been stabilized, by waiting until a long time has passed after the developer is supplied into the developing device, that is to say, by providing a long time for ageing. In such as case, a problem is involved where the agitating time becomes too long, requiring a long time before the control standard value is established. For this reason, this method is not adopted at all.